There have been a lot of problems with the "H" valve assembly. It can cause no cooling and evaporator freeze-up, and it can be intermittent. The system should be diagnosed before adding refrigerant. If it was fully charged, adding more refrigerant can allow liquid to enter the compressor. That's the kiss of death. Compressors can't handle a liquid without locking up.
There's a high and low pressure cutout switch on the system. If there's a leak and the compressor was allowed to run, it could draw in outside air and the moisture in it. That moisture would combine with refrigerant oil and become corrosive to metal parts. Since the compressor was running, it had sufficient charge.
Look on the hoses while the system is running and look for frost or ice buildup. If there is a restriction, that's the point where pressure drops and the refrigerant cools significantly. This is supposed to happen at the H-valve, not before or after it. The receiver / drier is a common source of restriction. If the H-valve is stuck open, the evaporator will be flooded with liquid and the cooling will take place in the hose going to the compressor, not in the passenger compartment.
If there is a leak, the compressor should stop cycling. An AC specialist will connect a pair of gauges to the system to evaluate its performance. It is impossible to tell how low the charge is except by recovering all of it and recharging the system with a measured amount. When some refrigerant vapor leaks out, system pressure drops. The lower pressure allows some of the liquid to vaporize and expand, so system pressure goes back up. With the system turned off, it will be under normal pressure until there's no liquid left to vaporize. That's why novices like you and me can't tell the state of charge, but experienced professionals can by watching the gauges during system operation.
If the AC hose going into the passenger compartment is warm and the one coming out is cold, look for water dripping onto the ground near the back of the engine. If there is, look for a problem with the temperature control door, especially if you have a computer-controlled system. If no water is dripping, check for ice buildup on the evaporator. Over-cooling from a stuck H-valve will lower the temperature below 32 degrees, and condensed humidity, which should drip onto the ground, will freeze on the evaporator blocking air flow. If you suspect this might be the case, run the AC on "Max" or "Recirculate". Max AC will recirculate cooled air from inside the vehicle that has already had the moisture removed. This will reduce ice buildup on the evaporator. Place a thermometer in one of the center duct outlets. On the coldest setting, duct temperature should be no less than around 40 degrees. If it's lower, check the sensing bulb on the evaporator or hose. It should be held in place with a black insulating material.
Look on the top of the receiver / drier for a sight glass. If you see bubbles, the system is low on charge. (Don't try this on a Ford product. Fords with a fully charged, properly operating system will still have bubbles in the sight glass).
Tuesday, March 24th, 2009 AT 10:45 PM